The present invention relates to an apparatus and a method for removing washcoat remaining in the channels of freshly coated monolithic or honeycombed catalytic carriers and the use of the apparatus in the production of catalysts having those carriers.
Monolithic or honeycombed substrates used for the preparation of exhaust gas purification catalysts are well known and can be made of ceramic or metal. Any such substrates can be treated in accordance with the herein described invention.
During the manufacture of monolithic or honeycombed catalysts, e.g. for the cleaning of automobile or industrial exhaust gases, the catalysts are usually coated with a film of a high surface-area, finely divided and porous ceramic metal oxide such as .gamma.-Al.sub.2 O.sub.3 for increasing the carrier surface. This coating deposited on the substrate is referred to in the industry as a "washcoat". This coating operation is usually performed by spraying or by an immersion method using a suspension of the metal oxide. An important phase of the manufacturing operation resides in the removal of the nonabsorbed washcoat remaining in the channels of the freshly coated, monolithic or honeycombed catalyst carriers. The channels cannot be clogged with excess washcoat as it will interfere with the stream of exhaust gas which passes through such devices in actual installations. Up to the present time, this removal step was carried out by blowing out the excess washcoat with compressed air, that is, the excess washcoat was removed and blown away from the solid body by means of an impulse or jet of an applied stream of air. This causes fine suspension particles in droplet form to be produced; i.e. an aerosol effect.
These particles can not be completely caught in storage tanks or the like with reasonable technical expense and effort so that in the course of the operating time not inconsiderable contaminations of the system occur as a result of being coated with dried washcoat. This can progress to such an extent that the mechanical functioning of the components of the system is adversely affected. This problem can be counteracted only with a considerable and periodic cleaning effort. In addition, especially finely atomized washcoat can result in the formation of an aerosol and adversely affect the surrounding. This must be counteracted by the installation of specially adapted exhaust devices.
As a result of these recognized problems in the industry, there was an urgent need for an improved method of operation which could also be automated.